Harsh climates encountered in areas, such as desert terrain in proximity to bodies of water, are known to be very difficult for Line-of-Sight (LOS) radio links because they cause frequent and large reductions in received signal strength. These signal strength variations occur because abnormal variations in the refractive index of the lower atmosphere cause multipath transmission and/or bending of beams emitted by the antenna. At frequencies below about 8 GHz and on paths having adequate clearance, these signal strength variations, otherwise known as time-varying fading, are generally of two main types: (1) atmospheric multipath interference, which occurs relatively rapidly and is caused by interference between two or more refracted rays arriving at the receiving antenna by different paths; and (2) reflection multipath interference, which occurs less rapidly and is due to interference between direct and ground-reflected waves. In terms of occurrence on a single radio link, multipath interference is the prevalent cause of degraded transmission reliability resulting from anomalous atmospheric structures. However, these two types of multipath fading can be present at the same time and the number of fades tends to increase with time due to atmospheric multipath increases along the path length.
The ability of a radio installation to withstand decreases in received signal strength is represented by its fade margin, i.e., the amount of power in dB that the average received signal strength exceeds the receiver threshold. Evaluation of known LOS radio systems shows that small fade margins of 4 to 6 dB were used in the past to accommodate for multipath fading. These fixed margins were originally disclosed in, "Radio Propagation Fundamentals," Bullington, K., The Bell System Technical Journal XXXVI(3), 1957, and were to provide for a link reliability of 90 percent for average climatic conditions. These margins, however, have since proven to be too small for difficult propagation environments and outdated in view of current LOS link engineering methodology, which includes parameters for climate, location, terrain, and path length.
Current LOS radio design requirements are based to a considerable degree on requirements and experience in Europe. However, propagation conditions in such countries as Germany are benign compared to those in the coastal areas of the Persian Gulf and in other warm and humid climates. A large body of research about LOS fading has been accumulated over the past 20 years, e.g., Vigants, "Number and Duration of Fades at 4 and 6 GHz," "Space-Diversity Engineering," and "Temporal Variability of Distance Dependence of Amplitude Dispersion and Fading," The Bell System Technical Journal 50(3)(1971) and 54(1)(1975) and Conference Record, International Conference of Communications, Amsterdam, the Netherlands (1984), respectively, and DCEC Engineering Publication (EP) 1-90, DCS Digital Line-of-Sight Link Design (1990). Much of this knowledge has its origin in the research performed for the engineering of commercial LOS links in the United States, where in the 1980s at least two-thirds of long distance communications traffic was routed over long-haul LOS microwave radio. The initiation of this propagation research was related to more efficient use of the frequency spectrum when the Federal Communications Commission reduced the number of frequency diversity protection channels from two to one in each frequency band. This required understanding of fading for a large variety of climatic conditions. The resulting models of fading and its countermeasures (frequency diversity and space diversity) permitted commercial link engineering to meet increasing reliability requirements related to increasing amounts of data transmission in any network. Subsequent introduction of microwave digital radio technology resulted in further propagation research on the in-band distortion of the frequency spectrum caused by multipath fading. The fading models developed from this research included meteorological variables. This permitted generation of an LOS propagation description for worldwide application when business opportunities arose related to providing microwave LOS communications to countries such as Saudi Arabia and other developing countries.
Today, the main focus is the single link propagation reliability defined as the percentage of time that the received signal strength is above the receiver's 10-5 Bit Error Rate (BER) threshold. Commercial systems employing modern digital radio typically require link reliabilities of 99.99 percent or better. These systems have been designed and are engineered on a per link basis to accommodate clear-air received signal decreases of 40 to 50 dB due to time varying fading of the received signal. It is important to note that commercial systems are designed for fixed-plant operation with generally good clearances and high antenna gains while military LOS radios are rapidly deployed and moved in operations where good sites and high-gain antennas are not the norm. The link engineering for these military digital radio systems, however, does not adequately take time-varying fading into account and does not incorporate results from the large body of recent published work on this topic.